A high-purity diphenyl carbonate is important as a raw material for the production of an aromatic polycarbonate, which is the most widely used engineering plastics, without using toxic phosgene. As a process for producing an aromatic carbonate, a process of reacting an aromatic monohydroxy compound with phosgene has been known from long ago, and has also been the subject of a variety of studies in recent years. However, this process has the problem of using phosgene, and in addition chlorinated impurities that are difficult to separate out are present in the aromatic carbonate produced using this process, and hence this aromatic carbonate cannot be used as a raw material for the production of the aromatic polycarbonate. Because such chlorinated impurities markedly inhibit the polymerization reaction in the transesterification method which is carried out in the presence of an extremely small amount of a basic catalyst; for example, even if such chlorinated impurities are present in an amount of only 1 ppm, the polymerization hardly proceeds at all. To make the aromatic carbonate capable of using as a raw material of a transesterification method polycarbonate, a troublesome multi-stage separation/purification processes such as enough washing with a dilute aqueous alkaline solution and hot water, oil/water separation, distillation and so on are thus required. Furthermore, the yield of aromatic carbonate decreases due to hydrolysis loss and distillation loss during this separation/purification processes. Therefore, there are many problems in carrying out this method economically on an industrial scale.
On the other hand, a process for producing aromatic carbonates through transesterification reactions between dialkyl carbonates and aromatic monohydroxy compounds are also known. However, such transesterification reactions are all equilibrium reactions. Since the equilibriums are biased extremely toward the original system and the reaction rates are slow, there have been many difficulties in producing the aromatic carbonate industrially in large amounts using this method. Two types of proposals have been made to improve on the above difficulties. One of these relates to development of a catalyst to increase the reaction rate, and many metal compounds have been proposed as the catalyst for the above type of the transesterification reactions. For example, Lewis acids such as transition metal halides and Lewis acid-forming compounds (see Patent Documents 1: Japanese Patent Application Laid-Open No. 51-105032, Japanese Patent Application Laid-Open No. 56-123948, Japanese Patent Application Laid-Open No. 56-123949 (corresponding to West German Patent Application No. 2528412, British Patent No. 1499530, and U.S. Pat. No. 4,182,726), Japanese Patent Application Laid-Open No. 51-75044 (corresponding to West German Patent Application No. 2552907, and U.S. Pat. No. 4,045,464)), tin compounds such as organotin alkoxides and organotin oxides (see Patent Documents 2: Japanese Patent Application Laid-Open No. 54-48733 (corresponding to West German Patent Application No. 2736062), Japanese Patent Application Laid-Open No. 54-63023, Japanese Patent Application Laid-Open No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110), Japanese Patent Application Laid-Open No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704), Japanese Patent Application Laid-Open No. 62-277345, Japanese Patent Application Laid-Open No. 1-265063), salts and alkoxides of alkali metals and alkaline earth metals (see Patent Document 3: Japanese Patent Application Laid-Open No. 57-176932), lead compounds (see Patent Documents 4: Japanese Patent Application Laid-Open No. 57-176932, Japanese Patent Application Laid-Open No. 1-93560), complexes of metals such as copper, iron and zirconium (see Patent Document 5: Japanese Patent Application Laid-Open No. 57-183745), titanic acid esters (see Patent Documents 6: Japanese Patent Application Laid-Open No. 58-185536 (corresponding to U.S. Pat. No. 4,410,464), Japanese Patent Application Laid-Open No. 1-265062), mixtures of a Lewis acid and protonic acid (see Patent Document 7: Japanese Patent Application Laid-Open No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501)), compounds of Sc, Mo, Mn, Bi, Te or the like (see Patent Documents 8: Japanese Patent Application Laid-Open No. 1-265064), ferric acetate (see Patent Document 9: Japanese Patent Application Laid-Open No. 61-172852), and so on have been proposed.
Since the problem of the disadvantageous equilibrium cannot be solved merely by developing the catalyst, as the other type of the proposals, attempts have been made to devise a reaction system so as to shift the equilibrium toward the product system as much as possible, thus improving the aromatic carbonate yield. For example, for the reaction between dimethyl carbonate and phenol, there have been proposed a method in which methanol produced as a by-product is distilled off by azeotropy together with an azeotrope-forming agent (see Patent Documents 10: Japanese Patent Application Laid-Open No. 54-48732 (corresponding to West German Patent Application No. 736063, and U.S. Pat. No. 4,252,737)), and a method in which the methanol produced as a by-product is removed by being adsorbed onto a molecular sieve (see Patent Documents 11: Japanese Patent Application Laid-Open No. 58-185536 (corresponding to U.S. Pat. No. 410,464)). Moreover, a method has also been proposed in which, using an apparatus in which a distillation column is provided on top of a reactor, an alcohol produced as a by-product in the reaction is separated off from the reaction mixture, and at the same time an unreacted starting material that evaporates is separated off by distillation (see Patent Documents 12: examples in Japanese Patent Application Laid-Open No. 56-123948 (corresponding to U.S. Pat. No. 4,182,726), examples in Japanese Patent Application Laid-Open No. 56-25138, examples in Japanese Patent Application Laid-Open No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110), examples in Japanese Patent Application Laid-Open No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704), examples in Japanese Patent Application Laid-Open No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501), examples in Japanese Patent Application Laid-Open No. 61-172852, examples in Japanese Patent Application Laid-Open No. 61-291545, examples in Japanese Patent Application Laid-Open No. 62-277345).
However, these reaction systems have basically been batch system or switchover system. Because there are limitations in the improvement of the reaction rate through catalyst development for such transesterification reactions, and the reaction rates are still slow, and thus it has been thought that the batch system is preferable to a continuous system. Of these, a continuous stirring tank reactor (CSTR) system in which a distillation column is provided on the top of the reactor has been proposed as the continuous system, but there are problems such as the reaction rate being slow, and a gas-liquid interface in the reactor being small, based on the volume of the liquid. Hence it is not possible to make the reaction ratio high. Accordingly, it is difficult to attain the object of producing the aromatic carbonate continuously in large amounts stably for a prolonged period of time by means of the above-mentioned methods, and many issues remain to be resolved before economical industrial implementation is possible.
The present inventors have developed reactive distillation methods in which such a transesterification reaction is carried out in a continuous multi-stage distillation column simultaneously with separation by distillation, and have been the first in the world to disclose that such a reactive distillation system is useful for such a transesterification reaction, for example, a reactive distillation method in which a dialkyl carbonate and an aromatic hydroxy compound are continuously fed into the multi-stage distillation column, and the reaction is carried out continuously inside the column in which a catalyst is present, while continuously withdrawing a low boiling point component containing an alcohol produced as a by-product by distillation and continuously withdrawing a component containing a produced alkyl aryl carbonate from a lower portion of the column (see Patent Document 13: Japanese Patent Application Laid-Open No. 3-291257), a reactive distillation method in which an alkyl aryl carbonate is continuously fed into the multi-stage distillation column, and the reaction is carried out continuously inside the column in which a catalyst is present, while continuously withdrawing a low boiling point component containing a dialkyl carbonate produced as a by-product by distillation, and continuously withdrawing a component containing a produced diaryl carbonate from a lower portion of the column (see Patent document 14: Japanese Patent Application Laid-Open No. 4-9358), a reactive distillation method in which these reactions are carried out using two continuous multi-stage distillation columns, and hence a diaryl carbonate is produced continuously while efficiently recycling a dialkyl carbonate produced as a by-product (see Patent document 15: Japanese Patent Application Laid-Open No. 4-211038), and a reactive distillation method in which a dialkyl carbonate and an aromatic hydroxy compound or the like are continuously fed into the multi-stage distillation column, and a liquid that flows down through the column is withdrawn from a side outlet provided at an intermediate stage and/or a lowermost stage of the distillation column, and is introduced into a reactor provided outside the distillation column so as to bring about reaction, and is then introduced back through a circulating inlet provided at a stage above the stage where the outlet is provided, whereby reaction is carried out in both the reactor and the distillation column (see Patent Documents 16: Japanese Patent Application Laid-Open No. 4-224547, Japanese Patent Application Laid-Open No. 4-230242, Japanese Patent Application Laid-Open No. 4-235951).
These reactive distillation methods proposed by the present inventors are the first to enable aromatic carbonates to be produced continuously and efficiently, and many similar reactive distillation systems based on the above disclosures have been proposed thereafter (see Patent Documents 17 to 32: Patent Document 17: International Publication No. 00/18720 (corresponding to U.S. Pat. No. 5,362,901), Patent Document 18: Italian Patent No. 01255746, Patent Document 19: Japanese Patent Application Laid-Open No. 6-9506 (corresponding to European Patent No. 0560159, and U.S. Pat. No. 5,282,965), Patent Document 20: Japanese Patent Application Laid-Open No. 6A41022 (corresponding to European Patent No. 0572870, and U.S. Pat. No. 5,362,901), Patent Documents 21: Japanese Patent Application Laid-Open No. 6-157424 (corresponding to European Patent No. 0582931, and U.S. Pat. No. 5,334,742), Japanese Patent Application Laid-Open No. 6-184058 (corresponding to European Patent No. 0582930, and U.S. Pat. No. 5,344,954), Patent Document 22: Japanese Patent Application Laid-Open No. 7-304713, Patent Document 23′: Japanese Patent Application Laid-Open No. 940616, Patent Document 24: Japanese Patent Application Laid-Open No. 9-59225, Patent Document 25: Japanese Patent Application Laid-Open No. 9-110805, Patent Document 26: Japanese Patent Application Laid-Open No. 9-165357, Patent Document 27: Japanese Patent Application Laid-Open No. 9-173819, Patent Documents 28: Japanese Patent Application Laid-Open No. 9-176094, Japanese Patent Application Laid-Open No. 2000-191596, Japanese Patent Application Laid-Open No. 2000-191597, Patent Documents 29: Japanese Patent Application Laid-Open No. 9-194436 (corresponding to European Patent No. 0785184, and U.S. Pat. No. 5,705,673), Patent Documents 30: International Publication No. 00/18720 (corresponding to U.S. Pat. No. 6,093,842), International Publication No. 01/042187 (corresponding to Published Japanese Translation of PCT Application No. 2003-516376), Patent Documents 31: Japanese Patent Application Laid-Open No. 2001-64234, Japanese Patent Application Laid-Open No. 2001-64235, Patent Document 32: International Publication No. 02/40439 (corresponding to U.S. Pat. No. 6,596,894, U.S. Pat. No. 6,596,895, and U.S. Pat. No. 6,600,061)).
Among the reactive distillation systems, the present applicants have further proposed, as a method that enables highly pure aromatic carbonates to be produced stably for a prolonged period of time without a large amount of a catalyst being required, a method in which a high boiling point material containing a catalyst component is reacted with a n active substance and then separated off, and the catalyst component is recycled (see Pa tent Documents 33: International Publication No. 97/11049 (corresponding to European Patent No. 0855384, and U.S. Pat. No. 5,872,275)), and a method carried out while keeping the weight ratio of a polyhydric aromatic hydroxy compound in the reaction system to a catalyst metal at not more than 2.0 (see Patent Documents 34: Japanese Patent Application Laid-Open No. 11-92429 (corresponding to European Patent No. 1016648, and U.S. Pat. No. 6,262,210)). Furthermore, the present inventors have also proposed a method in which 70 to 99% by weight of phenol produced as a by-product in a polymerization process is used as a starting material, and diphenyl carbonate can be produced by means of the reactive distillation method. This diphenyl carbonate can be used as the raw material for polymerization to produce aromatic polycarbonates (see Patent Documents 35: Japanese Patent Application Laid-Open No. 9-255772 (corresponding to European Patent No. 0892001, and U.S. Pat. No. 5,747,609)).
However, in all of these prior art documents in which the production of the aromatic carbonates using the reactive distillation method is proposed, there is no disclosure whatsoever of a specific process or apparatus enabling mass production on an industrial scale (e.g. more than 1 ton per hr), nor is there any description suggesting such a process or apparatus. For example, the descriptions regarding heights (H1 and H2: cm), diameters (D1 and D2: cm), the numbers of stages (N1 and N2), and the feeding rates of the raw materials (Q1 and Q2: kg/hr) for two reactive distillation columns disclosed for producing mainly diphenyl carbonate (DPC) from dimethyl carbonate and phenol are as summarized in the following table.
TABLE 1PATENTH1D1N1Q1H2D2N2Q2DOCUMENT600252066600252023153502.8—0.23055~1015 +0.621PACKING5005500.64008500.6231004—1.42004—0.8243005401.5—5250.72812002040866002520313334600—2066600—202235
In other words, the biggest continuous multi-stage distillation columns used when carrying out this reaction using the reactive distillation system are those disclosed by the present applicants in Patent Documents 33 and 34. As can be seen from Table 1, the maximum values of the various conditions for the continuous multi-stage distillation columns disclosed for the above reaction are H1=1200 cm, H2=600 cm, D1=20 cm, D2=25 cm, N1=N2=50 (Patent Document 25), Q1=86 kg/hr, and Q2=31 kg/hr, and the total amount of diphenyl carbonate produced was only approximately 6.7 kg/hr, which was not an amount produced on an industrial scale.
As methods for separating the diphenyl carbonate from the reaction mixture containing a diphenyl carbonate that has been produced through transesterification reaction and the like between a dialkyl carbonate and a phenol as a starting material as described above, and then purifying the diphenyl carbonate, crystallization methods, distillation methods and the like have been proposed. With regard to the distillation methods, three methods have been proposed. One is a method in which the diphenyl carbonate is obtained as a column top component from a distillation column; for example, there are:
I) a method in which the reaction mixture containing the catalyst is distilled as is in a batch type distillation column, and the diphenyl carbonate is obtained as the column top component (see example of Patent Document 10, example 2 of Patent Document 19);
II) a method in which the reaction mixture containing the catalyst is subjected to flash evaporation, and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is distilled in a distillation column for starting material recovery, and a catalyst-containing diphenyl carbonate is obtained as a column bottom material, and then this column bottom material is distilled in a purifying column, whereby the diphenyl carbonate is obtained as a column top component (see Patent Document 37: example 1 in Japanese Patent Application Laid-open No. 4-100824, Patent Document 38: Japanese Patent Application Laid-open No. 9-169704); and
III) a method in which the reaction mixture containing the catalyst is distilled in a distillation column (or evaporator), and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is subjected to continuous sequential distillation using a distillation apparatus comprising three columns, i.e. a light fraction separating column, a methyl phenyl carbonate separating column, and a diphenyl carbonate separating column, whereby diphenyl carbonate is obtained as a column top component (see Patent Document 25).
Another is a method in which the diphenyl carbonate is obtained as a column bottom component from a distillation column; for example, there is:
IV) a method in which the reaction mixture containing the catalyst is distilled in a distillation column, and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is distilled in a distillation column, and the diphenyl carbonate is obtained as a column bottom component (see Patent Document 31).
The other is a method in which the diphenyl carbonate is obtained as a side cut component from a distillation column; for example, there are:
V) a method in which the reaction mixture containing the catalyst is introduced into a third reactive distillation column, and further reaction and distillation are carried out, whereby the diphenyl carbonate is obtained as a side cut component from the reactive distillation column (see Patent Document 21);
VI) a method in which the reaction mixture containing the catalyst is subjected to flash evaporation, and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is introduced into a distillation column and distillation is carried out, whereby the diphenyl carbonate is obtained as a side cut component from the reactive distillation column (see Patent Documents 34 and 35, Patent Document 39: International Publication No. 92/18458 (corresponding to U.S. Pat. No. 5,426,207);
VII) a method in which the reaction mixture containing the catalyst is distilled in a first purifying column, and thus separated into a high boiling point material containing most of the catalyst and a low boiling point material, and then the low boiling point material is introduced into a second purifying column and distillation is carried out, whereby the diphenyl carbonate is obtained as a side cut component from the second purifying column (see Patent Document 40: Japanese Patent Application Laid-open No. 11-49727); and
VIII) a method in which diphenyl carbonate containing phenyl salicylate is introduced into a distillation column having the number of theoretical stages being from 5 to 15, and distillation is carried out at a column bottom temperature of not less than 150° C., whereby the diphenyl carbonate is obtained as a side cut component from the distillation column (see Patent Document 36: Japanese Patent Application Laid-open No. 9-194437 (corresponding to European Patent No. 0784048)).
However, it has been shown that various problems remain with such diphenyl carbonate separation/purification methods using these distillations. More specifically, the purity of the diphenyl carbonate obtained through the above I) is low, and moreover this is a batch process and hence is not suitable for mass production on an industrial scale. Regarding the above II), the method of Patent Document 37 is a batch method, and the diphenyl carbonate which was obtained through the method disclosed in Patent Document 38 contains a titanium catalyst, albeit in an amount of not more than 1 ppm, and hence is not suitable as a raw material for the production of a high-purity discolored polycarbonate. With the method of the above III), since the diphenyl carbonate is heated to a high temperature at the bottom of each of two of the distillation columns, i.e. the light fraction separating column and the methyl phenyl carbonate separating column, and is then subjected to a high temperature in the diphenyl carbonate separating column, the diphenyl carbonate is altered, bringing about a decrease in the purity and a decrease in the yield.
Moreover, the method of the above IV) in which the diphenyl carbonate is obtained from the column bottom is unsuitable, because the purity is low and hence a desired polycarbonate cannot be produced.
With the method of the above V), the reaction mixture containing the catalyst, the unreacted starting material and the impurities obtained from the bottom of the second reactive distillation column is introduced into the third reactive distillation column from an upper portion thereof, and the diphenyl carbonate is withdrawn from the side of the third reactive distillation column. Vapor or mist of the catalyst, the starting material, the impurities and the like may thus be entrained, and hence the purity of the diphenyl carbonate is low. With the method of the above VI), the amount of diphenyl carbonate produced is 6.7 kg/hr (example 3 of Patent Document 34) or 3.9 kg/hr (example 1 of Patent Document 35), which is not on an industrial scale. The method of the above VII) is a preferable process, but the amount of diphenyl carbonate produced is small at 2 kg/hr (example 8 of Patent Document 40), which is not on an industrial scale. Moreover, the method is carried out with the column top pressure in the first purifying column at a high vacuum of 200 Pa, and hence industrial implementation would be difficult, because a very large distillation column would be required so that the high vacuum could be maintained.
Moreover, with the method of the above VIII), although it is stated that the content of phenyl salicylate is reduced from 3000 ppm to 50 ppm (example 2 of Patent Document 36), nothing is stated whatsoever for other impurities. For example, even though the diphenyl carbonate is produced using the phosgene method in this example, and hence this is definitely a purification method for diphenyl carbonate containing chlorinated impurities, nothing is stated whatsoever with regard to the chlorinated impurities (which have an adverse effect on the polymerization to produce a polycarbonate and the properties of the polycarbonate even in an extremely small amount of only a few tens of ppb). With this method, such chlorinated impurities will not be separated out sufficiently, and hence it will not be possible to use the diphenyl carbonate as a raw material for a polycarbonate. This is as described in comparative example 1 (in which the alkali column is not used) of the purification method (in which after washing twice with alkaline hot water, washing with hot water is carried out, and then the diphenyl carbonate is dehydrated through distillation and then passed through a column filled with a solid alkali, before being subjected to reduced pressure distillation in the multi-stage distillation column) of Patent Document 41 (Japanese Patent Application Laid-Open No. 9-194437), which was filed more than one year after the filing of Patent Document 36.
Furthermore, in Patent Document 36, the temperature and time at which phenol starts to be distilled off in the case that reaction is carried out with bisphenol A are given as a method of evaluating the purity of the diphenyl carbonate obtained through the distillation, but evaluation of whether the diphenyl carbonate is suitable for polymerization cannot be carried out using this test method. This is because even for diphenyl carbonate of low purity such that a polycarbonate of the required degree of polymerization cannot be produced, the initial reaction in which phenol is eliminated occurs sufficiently. Moreover, since with this evaluation method, a large amount of 2.3 ppm of NaOH based on the bisphenol A is used as a catalyst, even for diphenyl carbonate containing, for example, 1 ppm of chlorinated impurities, an incorrect evaluation that the diphenyl carbonate is of high purity and is suitable as a raw material for a polycarbonate would be obtained. As stated earlier, the diphenyl carbonate containing 1 ppm of chlorinated impurities cannot be used as the raw material for the polycarbonate at all. In ordinary polymerization, since such a large amount of an alkaline catalyst is not used, this evaluation method is not suitable for evaluating the purity of diphenyl carbonate to be used for producing polycarbonate. Further, in Patent Document 36, there is no specific description whatsoever of purification of diphenyl carbonate that has been obtained using the transesterification method. Since the types and contents of impurities differ between diphenyl carbonate obtained through the phosgene method and diphenyl carbonate obtained using the transesterification method, it cannot be said that diphenyl carbonate of the same purity will be obtained through the same purification method. It thus cannot be said at all that diphenyl carbonate having the required purity for the raw material of the polycarbonate would be obtained through the purification method of Patent Document 36. Furthermore, the amount of purified diphenyl carbonate disclosed in Patent Document 36 is 0.57 kg/hr, which is not on an industrial scale.
A reaction mixture obtained through transesterification reaction between a dialkyl carbonate and a phenol as a starting material in the presence of a homogeneous catalyst generally contains various reaction by-products. In particular, if a diphenyl carbonate containing the amounts of high boiling point by-products having a higher boiling point than that of the diphenyl carbonate, such as phenyl salicylate, xanthone, phenyl methoxybenzoate, 1-phenoxycarbonyl-2-phenoxycarboxy-phenylene and the like which have not been reduced down to a sufficient level is used as the raw material of the transesterification method polycarbonate, then these high boiling point by-products will cause coloration and deterioration in properties. It is thus preferable to reduce the amounts of such impurities as much as possible. However, such high boiling point by-products are difficult to separate out, and with methods proposed hitherto, it has not been possible to reduce the amounts of such high boiling point by-products down to a sufficient level. In particular, there has been no proposal whatsoever of a process for the production on an industrial scale of not less than 1 ton/hr of a high-purity diphenyl carbonate required for the raw material of a high-quality and high-performance polycarbonate.